Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTS), referred to as Node Bs with regard to a universal mobile telecommunication system (UMTS), and a plurality of subscriber units, often referred to as user equipment (UE) in UMTS.
In a cellular network, such as UMTS, the power transmitted by a UE is regulated in order to minimise interference with other UEs. Typically, the output power generated by the radio frequency (RF) power amplifier (PA) in the UE will vary due to any number, or combination, of factors, such as the manufacturing process, operating temperature, supply voltage, antenna loading and other such factors. Thus, it often becomes necessary to measure the radio frequency transmit power at, or after, the PA output and to control the PA input, typically by controlling the gain of amplifiers located earlier in the amplifier chain, in response to this measurement. This feedback will allow power control regulation to compensate for variations in PA supply voltage, operating temperature & manufacturing process.
In addition, continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, intermodulation products can be generated in the non-linear radio frequency power amplifier. Thus, it is important to ensure any unwanted terms arising from the intermodulation are minimised and stay below a specified value. By modifying the bias to improve efficiency the PA becomes more non-linear, which in turn would lead to increased intermodulation problems. Specifically, in this field, there has been a significant amount of research effort in developing high efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier.
Linear modulation schemes require linear amplification of the modulated signal in order to minimise undesired out-of-band emissions. It is also known that non-linearities may create in-band distortion, which is typically measured by determining an error vector magnitude (EVM). Quantum processes within a typical RF amplifying device are inherently non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, the low efficiency is also recognised as being problematic for the base stations.
Furthermore, the emphasis in portable (subscriber) equipment is to increase battery life time. The emphasis for base station designers is to reduce power consumption, size, power dissipation, etc in order to reduce operating and equipment cost.
Hence, such operating efficiencies of the amplifiers used must be maximised. Thus, to achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number of linearising techniques exist, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion (APD) and these are often used to resolve the inherent trade off of linearity versus efficiency in wireless communication units.
One advantage of APD is that it offers excellent linearisation performance for a nonlinear, and consequently highly efficient, PA. One key feature for successful operation of APD is that off-line firmware (i.e. outside of the direct transmit path) is able to produce a reasonable starting estimation for the programmable gains of the APD loop, for example for the front-end digital domain logic elements and any programmable analogue attenuators that may exist.
However, it is also known that a subscriber communication unit may power up in a extreme conditions (e.g. at an extreme hot or cold temperature, or where there is a poor antenna mismatch, for example due to an object being located close to the antenna and thereby affecting the radiating field). Assuming that the firmware is phased under nominal conditions, then an extreme condition will cause the PA gain to vary significantly. Thus, the firmware then programs the variable gains in the line-up with non-optimal values.
In such extreme conditions, the transmitter loop gain may be significantly different from the pre-programmed nominal loop gain. Furthermore, in such extreme conditions, particularly with any antenna mismatch effect, any preloaded signal variable gain attenuator (SVGA) value applied at RF frequencies and digital attenuator (dig_attn) value applied in the digital domain of a forward path in the transmitted chain may be significantly removed from their optimal values. Here, there is an assumption in an APD system that other variable parameters, such as the gain in the receive (feedback) path hereinafter referred to as complex gain (cx_gain) values (albeit that the cx gain may be considered as real or complex values), variable gain attenuator (VGA) in a feedback path of the transmitter chain and look-up table (LUT) values used in a typical transmitter chain remain reasonably accurate. It is known that the elements in the forward path have a significant impact on the gain monitoring system. However, it is also known that the elements in the feedback path have an impact on any power accuracy and power backoff operation.
One consequence of the initial programming of non-optimal values is that any constant gain tracking (CGT) loop that is employed in the transmit chain may either hit a ceiling or a noise floor (depending on the type of gain variation). In such a case, if the SVGA and/or dig_attn value(s) is/are chosen to be too large, the CGT loop drives the digital signal into the noise floor, thereby causing signal-to-noise ratio (SNR) data recovery problems in the receiving communication unit. However, if the SVGA and/or dig_attn value(s) is/are chosen to be too small, the CGT loop drives the digital signal too high, thereby causing headroom problems, in that the signal to be transmitted is too close to its maximum level before exhibiting undesirable characteristics.
Following on from this, the APD loop can also act inappropriately, thereby causing the system to chronically fail in any one or more of the following standard's specifications: inaccurate power levelling, Adjacent Channel Power Ratio (ACPR), switching Output Radio Frequency Spectrum (ORFS), Adjacent Channel Leakage Ratio (ACLR), transient ACLR, and Error Vector Mean (EVM).
EP1570571 B1 provides one expensive solution to address the aforementioned problem, where any gain modification in a transmitter is effectively performed in the radio frequency (RF) domain, and complex initial calibration of the gain variation is required.